1. Field of the Invention
The present invention relates to stable polymer non-aqueous dispersions which comprise dispersed particles having nonionic and highly reactive surfaces, and are employable as anti-sag agents of coating compositions and aluminum pigment orientators or binder components of metallic coatings, and to a process for preparing the same and further to coating compositions formulated with the same.
2. Description of the Prior Art
Recently, coatings for automobiles have made a great advance in durability, providing high performances. This advance has been accompanied by increasing demands for higher qualities in aesthetic appearance of the coating films.
To accede to such demands, as for example disclosed in Japanese Patent Application Laid-open No. 53-133,234 (British Patent Specification No. 1,588,976), crosslinked polymer particles (may be hereinafter referred to as "polymer particles" or simply "particles") formed by emulsion polymerization processes or the NAD process have recently drawn particular attention from the fact that the polymer particles, when used in coating compositions, provide the coating compositions with structural viscosity based on interactions between the particles to display excellent flow control functions, such as metallic pigments orientation, sag prevention or the like.
The polymer particles to be used for such purposes are provided in the form of polymer non-aqueous dispersions that can be roughly classified into the following three types Namely, the first type is, as disclosed in Japanese Patent Applications Laid-open Nos. 58-129,065, 58-129,066, 58-129,069 (U.S. Pat. Nos. 4,530,946 and 4,461,870 are counterparts of these three), 60-250,068 and 61-42,579 (U.S. Pat. No. 4,563,372 is counterpart of these two), those obtained by the steps of: preparing crosslinked polymer particles by emulsion polymerization of .alpha.,.beta.-ethylenically unsaturated monomers in the presence of an amphoteric resinous surface active agent; then spray drying the resulting polymer particles to obtain dried particles; and re-dispersing the obtained dried particles in a solvent. This type is mainly used for the purpose of coating thickness increase. The second one is, as disclosed in U.S. Pat. Specifications Nos. 4,290,932 (Japanese Patent Application Laid-open No. 56-76,447), 4,377,661, 4,414,357, 4,477,536 and 4,598,111, is crosslinked polymer particles dispersed in a resinous solution which are prepared by the steps of: synthesizing by emulsion polymerization crosslinked polymer particles having ionic groups and polar groups on the surface; then, adding an azeotropic solvent to the resulting emulsion; and .circle. after coagulation dehydration and/or azeotropic dehydration, adding the resulting particles into the resin solution, .circle. after adding the emulsion to a solution polymerization batch of acrylic resins, conducting polymerization while azeotropically dehydrating or .circle. after mixing the emulsion with the resin solution, conducting azeotropic dehydration. The thus obtained particles dispersions have been used mainly for aluminum pigment orientators of metallic coatings by utilizing interparticle hydrogen bonding force and ionic bonding force. Further, the third one is, as disclosed in Japanese Patent Application Publication No. 16,147/1968 and Japanese Patent Applications Laid-open Nos. 53-133,234 and 54-150,439, those obtained through deposition by polymerizing .alpha., .beta.-ethylenically unsaturated monomers in organic solvents in the presence of amphipathic dispersion stabilizers. The obtained particles are used mainly for aluminum pigment orientators of metallic coatings and anti-sag.
However, the polymer non-aqueous dispersions obtained by those three processes have the following difficulties, respectively, and have not yet satisfied high coating performances as well as both qualities and appearances of coating films Namely, in the polymer non-aqueous dispersions obtained by the processes of the first type, since the resinous surface active agents and fragments of water-soluble polymerization initiators used in the manufacturing processes remain therein, ionic groups remain on surfaces of the particles, which deteriorate water resistance and moisture resistance of coating films or are liable to adsorb onto pigments, causing pigment aggregation when the dispersions are used in coating compositions.
Further, in the polymer non-aqueous dispersions obtained by the processes of the second type, fixation on surface of surface active agents is prone to occur during the emulsion polymerization, due to grafting on particle surfaces of lipophilic groups of the surface active agents, entanglement of molecular chains, or the like and, additionally, fragments of water-soluble polymerization initiators are also fixed on the particle surfaces to provide ionicity. Further, even if the solvent addition and coagulation dehydration process is used that is the most feasible to remove the surface active agents among the processes for dehydrating the emulsions, these oil-soluble emulsifiers are apt to remain in the organic phase, Removal of such surface active agents and fragments of water-soluble polymerization initiators is very difficult. As is referred to in Journal of Colloid and Interface Science, Vol. 52, p. 479 (1979) and U.S. Pat. Nos. 4,461,870 and 4,530,946, it is deemed impossible to remove completely surface active agents used in the emulsion polymerization from surfaces of polymer particles.
Therefore, difficulties are encountered such that, when coating compositions are formulated with the dispersions, water resistance and moisture resistance of coating films are deteriorated and, moreover, since ionic groups are fixed on particle surfaces, pigment aggregation is prone to occur when the dispersions are used in pigment systems, and additionally, they are liable to corrode metallic containers during storage.
Further, in the polymer non-aqueous dispersions obtained by the processes of the third type, since polymer particle forming materials are limited from the synthetic point of view, polar groups are difficult to introduce onto the particle surfaces, so that they are inferior in flow control function based on interactions between the particles to those obtained by the processes of the first and second types. Furthermore, since dispersion stabilizers liable to be solvated with aliphatic solvents are employed, the particles cannot exist with stability in usual coating solvents having a relatively high polarity, so that they have difficulties such that fusion, deposition, swelling, dissolution, etc. of dispersed particles are liable to occur.